In the prior art, the AA7000 series aluminum alloys, by virtue of their high strength, good corrosion resistance and high toughness, have been used in a wide range of applications. These alloys are often used in aircraft or aerospace components, automobile components, and in other high performance applications as plate and sheet products, extrusions, forgings, etc. These alloys generally contain zinc as a major alloying element and can also contain magnesium, copper and chromium. For example, the Aluminum Association limits for AA7075 are, in weight percent, 1.2-2.0% copper, 2.1-2.9% magnesium, 5.1-6.1% zinc and 0.18-0.28% chromium as major alloying elements.
Although the AA7000 series aluminum alloys offer significant benefits in terms of their various properties, these alloys can be difficult to hot work, particularly to extrude. Often times, these alloys are characterized as "hard to extrude" alloys.
As a result of the difficulty in hot working these types of alloys, the hot working rate, e.g., the extrusion speed, is generally conducted at relatively low rates, thereby compromising productivity.
As a consequence of this slow hot working rate or extrusion speed, aluminum alloy producers cannot take advantage of the economic savings associated with press quenching these types of materials. Press quenching is a method of combining solution heat treatment with extruding whereby the separate solution heat treating and quenching step used on heat treatable alloys is eliminated. In press quenching, the solution heat treatment that is typically performed in a separate step is combined as part of the hot working operation. Following press quenching, the material can be artificially or naturally aged to achieve the desired hardening effects. Press quenching and aging is generally designated as a T5 temper practice, compared to a T6 temper which includes a solutionizing step.
FIG. 1 schematically depicts various prior art processing techniques. The T5 temper practice is best represented by the homogenization step 1, the hot working and quenching step 3 and the aging step 5.
Prior art attempts to subject these types of alloys to press quenching or T5 temper practice by trying to optimize the quenching following hot working has not been generally successful to date. Quenching these "hard to extrude" alloys at the exit of a hot working operation, particularly an extrusion press, could result in cooling of the hot working tool, particularly the extrusion die. This cooling adversely affects the extrusion process. In other words, properties based on a T5 temper practice cannot be readily achieved by controlling the quenching following the hot working operation for these type of aluminum alloys.
As a result of the inability to achieve a T5 temper product in these "hard to extrude" alloys, these alloys are subjected to subsequent solution heat treating, quenching and aging (artificial or natural) as shown in steps 7 and 9 of FIG. 1. This practice is commonly referred to as a T6 temper practice and imparts the desired precipitation hardening effect in these heat treatable alloys.
While the solution heat treating, quenching and aging contribute to the improved mechanical properties of these types of alloys, other drawbacks can occur. For example, besides the higher energy costs required to perform a separate solution heat treating step, reheating the hot worked and quenched product can result in distortion of the hot worked product, particularly when the solution heat treating and quenching is conducted with the product in a horizontal configuration. Distortion to the product can be minimized using vertical quenching apparatus. However, this equipment is complicated and expensive and the use of such further adds to the overall cost of the T6 temper practice. Solution heat treating can also adversely affect corrosion resistance due to the occurrence of recrystallization, particularly at the workpiece surface. Grain growth at the surface presents a more conducive structure for corrosive attack, thereby potentially compromising the corrosion properties of a material at the expense of improved mechanical properties.
Other attempts to overcome the difficulty in press quenching these hard to extrude alloys have been made by increasing the hot working rate or speed. The theory behind this practice is to move the workpiece through the hot working operation at a faster rate so that the quenching can successfully retain the alloying elements in solution for a subsequent aging response. However, since the AA7000 series materials are hard to work or extrude, increasing the hot working rate results in surface tearing or galling of the product. Since the material is hard, excessive friction occurs at the material-hot working tool interface, thereby causing localized temperature increases and generation of surface defects.
Besides the problems noted above in AA7000 series alloys, another drawback exists with respect to applications requiring good corrosion resistance, particularly exfoliation corrosion resistance. In many of these types alloys, to obtain acceptable levels of exfoliation corrosion resistance, an additional aging or stabilizing practice, i.e., a T7 temper practice, is employed to attain the desired exfoliation corrosion resistance. This temper practice includes Step No. 11 in FIG. 1. As an example, AA7075 T73 for extrusions can include a two-stage aging process wherein the first stage heats the extrusion to 250.degree. F. (121.degree. C.) for 3-30 hours followed by a second heating to 325.degree. F. (163.degree. C.) for 15-18 hours. As another example, AA7150-T77511 requires that extrusions fabricated with this temper show exfoliation corrosion resistance equal to or better than level EB.
In view of the disadvantages noted above in processing AA7000 series alloys, a need has developed to provide improved corrosion resistance without a loss of mechanical properties. In addition, a need has developed to increase the productivity of hot working these types of alloys without the loss of surface quality. Finally, a need has developed to produce a T5 temper product for "hard to extrude" aluminum alloys.
The present invention solves these needs by providing a method of improving the corrosion resistance of AA7000 series alloy without the need for a T7 temper practice. In addition, the inventive method increases the hot working rate for these alloys while maintaining acceptable surface quality. Finally, the inventive method attains T6 temper practice properties in an AA7000 series alloy by following a T5 temper practice. An integral part of the inventive methods described above is the utilization of a starting material having a globular and non-dendritic microstructure for the hot working step.
The formation of globular microstructures in aluminum alloys as a precursor to subsequent shaping is known. U.S. Pat. No. 5,730,198 to Sircar discloses such a method. Other techniques to form globular and non-dendritic microstructures include such methods as SIMA (strain-induced melt activated), magnetohydrodynamic (MHD) casting and rheocasting as disclosed in the Metal Handbook, volume 15, 9.sup.th edition, 1988 in an article entitled "Semisolid Metal Casting and Forging by Kenny, et al." Each of these disclosures is herein incorporated by reference. While the formation of globular microstructures in aluminum alloys is known, the prior art fails to recognize the improvements in processing parameters and corrosion resistance as described above using a starting workpiece having such a microstructure.